1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus suitably applicable with microfilm readers or microfilm reader-printers. The image forming apparatus is arranged to observe an enlarged image projected onto a screen surface by a projection lens from a reduced projection image of microfilm or the like and to project and record the enlarged projection image on a surface of a recording medium (a light-receiving medium) such as a line sensor (CCD) or a photosensitive member.
2. Related Background Art
The normal image forming apparatus including the microfilm readers have an observation system for enlarging and projecting the projection image (image information) of a microfilm or the like, which includes a record reduced images, onto the screen surface by the projection lens and a recording system (or a reading system) for projecting and recording the projection image on the surface of line sensor or photosensitive member by the projection lens.
Projection magnifications of the conventional microfilm reader-printers and the like as the image forming apparatus of this type range from 7 times to 50 times and the screen-side NA (numerical aperture) of the projection lens are set to be identical so as to maintain brightness constant on the screen regardless of the projection magnifications. Therefore, the film-side NA are small at low projection magnifications, whereas the film-side NA are large at high projection magnifications.
Many illumination optical systems of such microfilm reader-printers and the like use a collector-lamp-incorporated mirror (a lamp with a collector mirror) as a light source in order to increase the efficiency of light source. This collector-mirror-incorporated lamp was arranged so that a hole was provided at the center of the collector mirror to set the lamp therein. Therefore, the light near the optical axis was not used and the illumination efficiency was lowered by the decrease of illumination of that light.
A variety of proposals have been made heretofore, for example in the bulletin of Japanese Laid-open Patent Application No. 2-244134, the bulletin of Japanese Laid-open Patent Application No. 3-25744, and so on, as to the image forming apparatus (the illumination systems) for relieving the influence of the hole at the center of this collector mirror. The illumination systems proposed in these bulletins were arranged to move a small-size lens of a condenser lens system against movement of the pupil position of the projection lens.
In recent years, the image forming apparatus of microfilm reader-printers and the like are required to permit digital reading with a recording medium (solid state image sensing device) such as a line sensor (CCD), for example, so as to take in an image as data of personal computer. Since this line sensor has narrow latitude of quantity-of-light sensitivity, for example different from the photosensitive drum, it has a problem that the line sensor reads even small quantity-of-light unevenness. It is possible to correct the sensitivity of quantity-of-light unevenness for each pixel of line sensor, but, for example with use of a rotation prism (an image-inverting prism), there are problems that rotation of the rotation prism changes the direction of the quantity-of-light unevenness and that it is difficult to handle fine quantity-of-light unevenness practically because of limitations of capacity of memory devices for storing sensitivity correction values or the like. Therefore, the quantity-of-light unevenness and change of absolute quantity of light are desirably as little as possible.